For the problems I mentioned above it now really seem's to be my own
fault. For the case of the DB corruptions I could now reproduce it. In
this case I'm loading 500.000 entries with ldapadd into the directory. An
entry consists of about 23 attributes. After about 440.000 entries I
get the following messeages:

If you load the 500,000 entries via slapadd, do you see the same thing?
ldapadd as a process to add large amounts of entries isn't generally
advisable. Perhaps there is a memory leak somewhere. Have you used gdb to
set a breakpoint when this error occurs, and do a backtrace? Some
profiling of the running process to see its memory usage? It is hard to
tell here if the issue is in OpenLDAP or BDB. Although I could see a
memory leak causing a loss of resources...